During their fabrication process, ICs (integrated circuits) often incur defects due to minor imperfections in the process or in the semiconductor material. For that reason, ICs are usually designed to contain redundant circuit elements, such as spare rows and columns of memory cells in semiconductor memory devices, e.g., a DRAM (dynamic random access memory), an SRAM (static random access memory), or an embedded memory. Such devices are also designed to include particular laser-severable links between electrical contacts of the redundant circuit elements. Such links can be removed, for example, to disconnect a defective memory cell and to substitute a replacement redundant cell. Similar techniques are also used to sever links in order to program or configure logic products, such as gate arrays or ASICs (application-specific integrated circuits). After an IC has been fabricated, its circuit elements are tested for defects, and the locations of defects may be recorded in a data file or defect map. Combined with positional information regarding the layout of the IC and the location of its circuit elements, a laser-based link processing system can be employed to remove selected links so as to make the IC useful.
A typical link processing system adjusts the position of the laser beam spot on a semiconductor wafer by moving the wafer in an XY plane underneath a stationary optics table, which supports a laser and other optical hardware. The wafer is moved underneath in the XY plane by placing it on a chuck that is carried by a motion stage. A typical wafer contains a number of dies, each containing an IC. Circuit elements within an IC that are typically arranged in a regular geometric arrangement, as are the links between those elements. The links usually lie in regular rows in groups which are termed “link banks,” having an approximately uniform center-to-center pitch spacing. To remove selected links in a link bank, a beam spot (i.e., the position at which the laser beam's propagation path axis intersects the wafer workpiece) continuously advances along the link bank at an approximately uniform speed while the laser emits pulses to selectively remove links. The laser is triggered to emit a pulse and thereby to sever a link at a selected target position when the laser beam spot is on the target position. As a result, some of the links are not irradiated and left as unprocessed links, while others are irradiated to become severed. The process of progressing down a row of links and severing selected links with a laser pulse is termed a “link run.”
The laser in a typical link processing system is a Q-switched cavity laser that generates pulses at a pulse repetition frequency (“PRF”). When the Q switch is closed, energy builds up in the laser cavity. When the Q switch is opened, the built-up energy is released as a laser beam pulse. The process of opening and closing the laser's Q switch occurs repeatedly at a rate equal to the PRF.
Problems can arise if the Q switch is closed for an undesirably long period of time resulting in an undesirably large amount of energy accumulated in the laser cavity. The accumulated energy will eventually be released when the Q switch is opened. The resultant laser pulse can have greater energy than is desired to accomplish certain tasks and may damage the workpiece, components of the optics path, or the laser itself. Furthermore, leaving the Q switch closed for a period of time longer than occurs during nominal PRF operations may perturb the thermal equilibrium of the laser cavity or any wavelength-altering optic elements along the laser beam's propagation path. This may undesirably alter the laser pulse properties.